Defending champion Magician has been ruled out of the Breeders' Cup Turf at Santa Anita.
The Aidan O'Brien-trained colt stormed past The Fugue to win the 1-1/2 mile (2400m) Turf last year with Ryan Moore aboard.
O'Brien's son Joseph was to be in the saddle on the same track on Saturday, but the trainer said Magician would be scratched with lameness in his right foreleg.
"When he came out for a trot this morning he was found to be acutely lame on his off-fore," O'Brien said.
"It was an instant decision to take him out of the race."
The Breeders' Cup Turf was slated to be Magician's final career start and the plan was to retire him to Coolmore's Ashford Stud in Kentucky.